1. Field of the Invention
The present invention relates to a technique of processing image data supplied to an image display apparatus.
2. Description of the Related Art
A liquid crystal display (LCD) panel is generally used for an image display of cellular phones. The LCD panel switches on and off the driving voltage applied to liquid crystal cells arranged in a matrix, so as to vary the transmission of the liquid crystal and display two-tone images, such as letters, characters, and pictures. The cellular phones have been advanced significantly to attain multiple functions, and some models are accessible to the Internet. With such advance, the multiple tone expression is required for the LCD panel of the cellular phones to allow display of many pieces of information. A color LCD panel has been applied for the cellular phones to enable multiple tone expression of color images.
Image data read with an input device, such as a scanner or a digital camera, and image data designed on the computer are generally RGB data (8 bits for each of R, G, and B and 256 tones in total). The LCD panel mounted on the cellular phone has a less number of expressible tones in each cell than the number of tones included in original image data. Color reduction is accordingly carried out. It is assumed here that the LCD panel provides expression of eight tones. FIG. 20 shows a mapping of 256 tones of image data to 8 tones. The procedure equally divides the 256 tones of image data into 8 divisions and successively allocates the tone values in each division to each “display tone value” expressible by the LCD panel. This accomplishes color reduction from 256 tones to 8 tones. For example, the pixels having the input tone value of 190 are unequivocally mapped to the display tone value of 5. This method is called ‘simple color reduction’.
The multiple tone expression on the LCD panel may be attained by stepwise setting the effective driving voltage applied to liquid crystal cells and stepwise regulating the transmittance of the liquid crystal. There are two known settings for the driving voltage of the LCD panel. FIGS. 21A and 21B show voltage-transmittance characteristics (V-T characteristics) of the LCD panel, that is, the transmittance of the liquid crystal against the effective driving voltage.
The first setting has the transmittance at equal intervals as shown in FIG. 21A. As is known to those skilled in the art, the LCD panel has non-linear V-T characteristics. The technique thus regulates the effective driving voltage to equalize the intervals of the transmittance by taking advantage of the pulse width modulation. The effective driving voltage corresponds to the display tone value expressible by the LCD panel, so that the display tone value and the output lightness hold a linear relationship.
The second setting has the effective driving voltage at equal intervals as shown in FIG. 21B. One picture screen consists of a plurality of frames. The technique controls ON and OFF the driving voltage with regard to each pixel frame by frame to allow multiple tone expression. This setting, however, causes a variation in interval of the transmittance, that is, a variation in interval of the lightness expressible by the LCD panel. The display tone value expressible by the LCD panel and the output lightness accordingly have a non-linear relationship. For example, when the LCD panel is driven in the range of the effective driving voltage shown in FIG. 21B, the expressible lightness has wide intervals in an intermediate tone region, while having narrow intervals in both a low tone region and a high tone region.
By taking into account the color reproducibility, the LCD panel of the pulse width modulation type has mainly been applied for the cellular phones.
The pulse width modulation, however, has a large rate of power consumption. From the viewpoints of the extended life of the battery and energy saving, such large power consumption is serious problem in cellular phones having small battery capacities. The Frame Rate Control (FRC) with a smaller rate of power consumption has thus also been applied for the cellular phones.
As discussed previously, the Frame Rate Control (FRC) has non-linear display characteristics and suffers significant deterioration of the picture quality due to that. The deterioration of picture quality is especially remarkable in natural images that have a large percentage of image data in the intermediate tone region. For example, in the case of displaying an image of ‘sky’ or ‘flesh’ having continuously varying tone values, the pixels of identical lightness (display tone value) collectively appear in a specific area where pixels adjoining to each other in the original image data have close tone values. Even a one-step difference in display tone value leads to a significant difference in lightness. The false-contour thus appears on the boundary between pixels of different display tone values. It is difficult to improve such deterioration of the picture quality by the hardware configuration.
A liquid crystal display apparatus with the LCD panel generally has an electronic volume for adjusting the display contrast. The electronic volume is individually adjusted to maximize the display contrast of the LCD panel. FIG. 22 is a graph showing adjustment of the display contrast of the LCD panel with the electronic volume. For example, at the setting of the electronic volume equal to ‘1’, the voltages at the ON state and the OFF state of the driving voltage of the LCD panel are respectively V1on and V1off. The transmittances are T1on and T1off. At the setting of the electronic volume equal to ‘2’, the voltages at the ON state and the OFF state of the driving voltage of the LCD panel are respectively V2on and V2off. The transmittance are T2on and T2off. There is a relationship of V1on/V1off=V2on/V2off=fixed. T1on-T1off or T2on-T2off corresponds to the contrast.
The contrast of the LCD panel varies according to the working environments (temperature and brightness) and the settings (ON-OFF state of the backlight). For example, the temperature characteristic of the LCD panel affects the contrast thereof. At low environmental temperatures, the transmittance of the LCD panel is lowered to reduce the contrast. At high environmental temperatures, on the contrary, the transimittance of the LCD panel is raised to enhance the contrast. Such a variation in contrast may deteriorate the picture quality of the resulting displayed images.